Feeler gauge

A feeler gauge is a set of thin metal blades, each with a known thickness, used to measure small gaps or clearances between surfaces.

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Clearance: The space between two components or surfaces, often specified to permit movement or prevent contact. A feeler gauge measures this space by testing which blade fits.
Valve clearance: The gap between an engine valve's stem and its actuating mechanism when the valve is closed. Mechanics use feeler gauges to set this clearance in many engines.
Thickness: The distance between opposite surfaces of a solid object. Each blade's thickness is the known dimension used to compare with a gap.
Wire gauge: A measuring tool or scale used to determine a wire's diameter or thickness. It measures wire dimensions, not the gap between two surfaces.
Interference fit: A fit in which the mating dimensions overlap, requiring force or deformation to assemble the parts. It contrasts with the free clearance a gauge blade is meant to measure.
Spark plug gap: The distance between a spark plug's center and ground electrodes. A blade gauge checks this gap before ignition performance is affected.
Metrology: The science of measurement, including its units, methods, standards, and uncertainty. Feeler gauges are simple instruments whose use depends on measurement principles.
Radius gauge: A set of templates used to identify or check the radius of a curved profile. Its shaped leaves test curvature rather than clearance thickness.
Tolerance: The permitted variation in a specified dimension or other measurable property. Gauge readings are compared with the allowed clearance range.
Piston ring end gap: The space between the ends of a piston ring when it is placed in a cylinder bore. A feeler gauge measures this assembly clearance during engine work.